A: Yes, that is true. Air circulation evaporates water that inevitably condenses in walls in spring and fall. Please see my other answers to people’s questions on moisture on this page. However, it is almost impossible to seal an existing house so well that you get moisture problems. It happens far more often on new construction, particularly on houses built to the Passive House standard with requires almost no infiltration of air. The best way to deal with condensation in a house is to control the humidity which is easy to do with heat pumps. Please see my other answers on condensation and moisture problems in houses on this page. Condensation in houses is the most insidious problem in houses today.
Q: How toxic are the various kinds of insulation?
A: This question mostly arises in conjunction with formaldehyde in foam insulations. Formaldehyde is not used in making cellulose insulation which is made from shredded newspaper, fiberglass which is made from fibers of melted glass or rockwool which is made from fibers of melted rock.
Strictly speaking spray foam is not toxic. But, if your house catches fire, it does burn with a thick black smoke that can be toxic and this is one reason I avoid it if I can and use fiberglass or rock wool (neither of which can burn) instead.
Also, the gas that is used to make the bubbles in spray foam is usually a gas called an HFC (or hydrofluorocarbon) that, although, strictly not toxic itself, is 1,000 stronger than CO2 in causing global warming, so it is, in that sense, toxic.
Q: How often does insulation need to be replaced?
A: Spray foam will last as long as the building. Dense-packed cellulose and fiberglass tend to settle down over time and so become less effective. However, taking off your siding or drywall is expensive and the reduction in heating bills will likely not pay for the cost of this work even if your insulation is in poor shape. It is better to add insulation opportunistically see Chapter 2 in Zero Carbon Home.
Q: Another question please. Our contractor is saying that spray foam insulation on the basement ceiling is more efficient. Do you have any thoughts about this recommendation?
A: Hi XXXXX, spray foam has R5 insulation per inch whereas fiberglass is R3, so in that sense spray foam is more efficient.
However, spray foam sticks to the joists like glue (that’s the idea) so you can’t get it out if you need to get access to a pipe or wire. For this reason I do not recommend spray foam for the ceiling of a basement.
Also, spray foam is more expensive per inch and per R value than fiberglass. So, economically, sprayfoam is less efficient than fiberglass.
And, finally, the gas used to blow the bubbles in spray foam is 1,000 times more potent than CO2 at causing global warming.
And, finally finally, spray foam is flammable whereas fiberglass is not.
For all these reasons, I prefer fiberglass to spray foam for insulating the ceiling of a basement.
And, really finally, fiberglass is porous to air which means that if some water does get into the fiberglass (like from an overflowing bath) it will evaporate and dry out. Spray foam is water tight which means any water falling onto the spray foam from above will sit there like a puddle with no ventilation and hence will take much longer to evaporate and dry out. This is risk for mold, rot and asthma for the people in the house.
Q: You insulated your basement ceiling because it’s unfinished and unheated? If you heat your basement, it wouldn’t make sense to insulate the basement ceiling, right?
A: Correct. Go to the top of the class. If you use the basement as living space, you need to insulate the walls and the floor of the basement.
Q: Once the air leakage is reduced, are you installing hrv or erv to provide fresh air?
A: We did not. It is almost impossible to seal an existing house so well that it gets stuffy and needs mechanical ventilation. However, on tightly sealed new construction you really do need one, it is not an option.
Q: Might you only insulate the ceiling of the utility side of the bsmt if half of the bsmt is conditioned space (family room)?
A: Yes, this is a good idea – insulate the basement ceiling where you don’t mind the basement getting a few degrees colder. If the basemetn space is living space then it is better to not insulate the basement but instead insulate the walls and add a thick carpet with a thick underlay pad.
Q: We need a new roof. We are thinking of metal. What do you think about metal?
A: I think standing-seam metal roofs are the best long-term investment for making a roof. They play especially well with solar panels because the panels are clamped to the seams, so no penetration is required on a sloped roof.
Q: My attic floor is insulated but not the ceiling. I had heard conflicting things about insulating both. What is your advice?
A: Insulating contractors I have spoken with recommend removing the insulation on the floor before adding insulation to the sloped sides of a roof. This makes sense if the insulation has mold growing or has mice living in it. But other than that, I do not see what harm in having more insulation. Often, the reason people are insulating the sloped sides of a roof when the floor of the attic is already insulated is because they are adding a heat pump air handler unit to the attic. It is essential to insulate the sloped parts of a roof when adding an air-handler unit to an attic or the heat from the air handler in winter will melt the snow and cause icicles or ice dams. The only circumstance that I could see that would warrant removing the insulation on the floor would be if the attic now gets warmer in winter than the house below it. Then it would make sense to allow the heat from the air handler to be conducted down into the house. These circumstances would be quite likely if you were adding a very thick layer of spray foam to the sloped parts of the attic.
Q: Our 100-year-old walls are very drafty–quite icy to the touch in winter. In multiple home energy audits, we’ve been told it is impossible to blow insulation into them. If we replaced the siding, would that allow us to add insulation outside the existing walls. Likewise, we definitely don’t have 10 inches to spare in the attic. If we replace the roof, is it possible to add a layer of insulation?
A: If you replace the roof, you can add a layer (or two) of ISO board to the roof on the outside of the plywood sheathing. This is what we did on our flat roof. The DOE recommends that roofs be insulated to at least R49 in MA. This would require 5 layers of ISO board. In my opinion this is spending too much money given how much you can save – insulation has diminishing returns to investment. After about R35-R40 the savings from the next layer of insulation rarely pay for the extra cost. Spend the money you save on a good air-sealing job.